Water deficit is a common component of many plant stresses. Water deficit occurs in plant cells when the whole plant transpiration rate exceeds the water uptake. In addition to drought, other stresses, such as salinity and low temperature, produce cellular dehydration (McCue and Hanson, 1990).
Heat stress often accompanies conditions of low water availability. Heat itself is seen as an interacting stress and adds to the detrimental effects caused by water deficit conditions. Evaporative demand exhibits near exponential increases with increases in daytime temperatures and can result in high transpiration rates and low plant water potentials (Hall et al., 2000). High-temperature damage to pollen almost always occurs in conjunction with drought stress, and rarely occurs under well-watered conditions. Thus, separating the effects of heat and drought stress on pollination is difficult. Combined stress can alter plant metabolism in novel ways; therefore, understanding the interaction between different stresses may be important for the development of strategies to enhance stress tolerance by genetic manipulation.
“Chilling sensitivity” describes many types of physiological damage produced at low, but above freezing, temperatures. Typical chilling damage includes wilting, necrosis, chlorosis or leakage of ions from cell membranes. The underlying mechanisms of chilling sensitivity are not completely understood yet, but probably involve the level of membrane saturation and other physiological deficiencies. By some estimates, chilling accounts for monetary losses in the United States second only to drought and flooding.
Based on the commonality of many aspects of cold, drought, and salt stress responses, genes that increase tolerance to cold or salt stress can also improve drought stress protection. In fact, this has already been demonstrated for some transcription factors, such as AtCBF/DREB1, and for other genes such as OsCDPK7 (Saijo et al., 2000), or AVP1 (a vacuolar pyrophosphatase-proton-pump, Gaxiola et al., 2001).
This study identifies polynucleotides encoding another group of transcription factors that can improve tolerance to cold and/or water deficit conditions. The protein sequences of the invention, which belong to the CCAAT-binding family of transcription factors, have been introduced into transgenic plants that were then found to have greater tolerance to cold and water deficit stress than control plants. Thus, important polynucleotide and polypeptide sequences for producing commercially valuable plants and crops as well as the methods for making them and using them were discovered. Other aspects and embodiments of the invention are described below and can be derived from the teachings of this disclosure as a whole.